The present invention relates to a device for arranging conductive particles in a preselected pattern for the connection of electric circuit boards or electric parts. More particularly, the present invention is concerned with a device for surely and efficiently transferring solder bumps to the electrode pads of a semiconductor chip or the leads of a TAB (Tape Automated Bonding) tape, and a conductive particle transferring method using the same.
It is a common practice with, e.g., LSI (Large Scale Integration) circuits and LCDs (Liquid Crystal Displays) to connect electric circuit boards by using conductive particles. After electric conduction has been set up between the circuit boards by the conductive particles, the circuit boards are fixed by an adhesive. Specifically, after the conductive particles have been arranged on either one of the circuit boards, an adhesive is applied and then set after the alignment of electrodes. To arrange the particles on the circuit board, they may be simply sprayed, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 2-23623 and 3-289070.
With the spraying scheme, however, it is difficult to control the positions and the number of the particles on the electrodes. Particularly, when the electrodes are arranged at a fine pitch, the particles are apt to short the electrodes or to render the connection resistance irregular due to the irregular number thereof on the electrodes. Although the particles may be arranged on the electrodes while having their positions controlled, such an approach needs a sophisticated control system.
For the electrical connection of the electrode pads of a semiconductor chip and outside leads, a wire bonding system, a TAB system and a flip-chip bonding system are typical systems available at the present stage of development. The TAB system and flip-chip bonding system each uses conductive particles in the form of solder bumps (simply bumps hereinafter) for electrical connection. Specifically, in the TAB system, bumps intervene between the electrode pads of a semiconductor chip and the film-like leads of a TAB tape. In the flip-chip bonding system, bumps intervene between the electrode pads of a semiconductor chip and the leads of a circuit board.
Today, the following methods are extensively used to form bumps. In one method, the exposed portions of electrode pads provided on a semiconductor chip are covered with barrier metal. After a solder film pattern has been formed on the barrier metal, reflow and annealing are effected in order to cause the solder film to shrink on the barrier metal due to its own surface tension. In another method, bumps are formed on the electrode pads one by one by a wire bonder. Recently, a transfer bump method has been proposed which is advantageous over the above direct methods from the step and cost standpoint. The transfer bump method forms bumps on an exclusive transfer substrate by an electrolytic plating scheme. The bumps on the transfer substrate are aligned with the leads of a TAB tape in the TAB system or with the electrode pads of a semiconductor chip in the flip-chip bonding system. Then, the bumps are bonded by heat and transferred to the leads or the electrode pads. It is not too much to say that the transfer bump method has broadened the applicable range of the TAB system.
However, the problem with the bumps formed by the electrolytic plating scheme is that they have flat surfaces and cannot be evenly transferred unless they have exactly the same height. In light of this, Japanese Patent Publication No. 7-27929 discloses a device capable of arranging spherical bumps on a transfer substrate. However, while the electrolytic plating scheme is capable of defining positions for forming the bumps beforehand, the spherical bumps are produced at random. Therefore, the key to the spherical bump scheme is how efficiently the bumps can be arranged in preselected positions. For the efficient arrangement of the bumps, the above document teaches that the diameter of the spherical bumps is strictly controlled. However, because the diameter of the bumps decreases with a decrease in the pitch of the electrode pads or that of the leads, it is extremely difficult to provide the bumps with the same diameter. As a result, the accuracy required of the flatness of the leads of a TAB tape, the flatness of a bonding tool and the parallelism of a transfer substrate and a TAB tape increases. The adjustment of such factors will become more difficult in the future in parallel with the progress of dense mounting.